1. Field of the Invention
This invention relates to a method of extracting residual vegetable oil from spent bleaching clay used in processing of such oil and in processing of the spent bleaching clay with the oil removed for reuse in the form of an activated carbon/clay mixture.
2. Prior Art Related to the Disclosure
Unrefined vegetable oils, such as soybean oil, cottonseed oil, palm oil and coconut oil contain color bodies and impurities which, unless removed, render the oil commercially unsuitable. Such unrefined oils are generally refined by neutralizing them to reduce the fatty acid content thereof and then running them through a bleaching and filtering operation where the neutralized oil is heated to 100.degree.-120.degree. C. with a bleaching clay, such as Fuller's earth, a naturally porous aluminum silicate, for about 15 minutes, after which the oil is filtered. The clean oil recovered from the filtration operation may be further polished and processed. The spent clay left behind is, by nature, an absorbent and retains from 15-50 percent by weight of the vegetable oil. The amount of retained oil in the spent bleaching clay, generally discarded as waste material, is significant if it could be extracted in a convenient, practical and economical manner. In Malaysia, where palm and coconut oil are produced as a crop, processed and refined, there is an estimated 2 percent loss of oil due to its retention in the spent clay. 1980 estimates of the tonnage of palm oil to be exported from Malaysia are 2,568,000 tons. If all of this exported oil were bleached by use of such bleaching clays, about 51,360 tons of oil would remain in the bleached clay, an amount sufficient to justify extraction if an economical extraction process were available.
Pressing of the spent clay to remove the oil does not yield usable oil. Although it would be expected that solvent extraction of the oil from the spent clay would be practical, fine particles of the spent clay are carried along with the oil during solvent extraction and cannot be removed economically by filtration. If the spent clay containing the residual oil is compressed or pelletized to prevent carrying of the fine particles of the clay along with the oil during extraction, the solvent extraction process reduces the pellets to fine powder before extraction is complete, again making it both economically and practically infeasible to remove the clay particles in the oil by filtration.